Net Community Production, Dissolved Organic Carbon Accumulation, and Vertical Export in the Western North Atlantic

Nicholas Baetge,Jason R. Graff,Michael J. Behrenfeld,Craig A. Carlson

doi:10.3389/fmars.2020.00227

Nicholas Baetge, Jason R. Graff + Show 2 more

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https://doi.org/10.3389/fmars.2020.00227

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Abstract

The annual North Atlantic phytoplankton bloom represents a hot spot of biological activity during which a significant fraction of net community production (NCP) can be partitioned into dissolved organic carbon (DOC). The fraction of seasonal NCP that is not respired by the heterotrophic bacterial community and accumulates as seasonal surplus DOC (∆DOC) in the surface layer represents DOC export to the upper mesopelagic zone, and in the North Atlantic this is facilitated by winter convective mixing that can extend to depths >400 m. However, estimates of ∆DOC and vertical DOC export for the western North Atlantic remain ill-constrained and the influence of phytoplankton community structure on the partitioning of seasonal NCP as ∆DOC is unresolved. Here, we couple hydrographic properties from autonomous in situ sensors (ARGO floats) with biogeochemical data from two meridional ship transects in the late spring (~ 44 – 56˚N along ~ -41˚W) and early autumn (~ 42 – 53˚N along ~ -41˚W) as part of the North Atlantic Aerosols and Marine Ecosystems Study (NAAMES). We estimate that 4 – 35% of seasonal NCP is partitioned as ∆DOC and that annual vertical DOC export ranges between 0.34 – 1.15 mol C m-2 in the temperate and subpolar North Atlantic. Two lines of evidence reveal that non-siliceous picophytoplankton, like Prochlorococcus, are indicator species of the conditions that control the accumulation of DOC and the partitioning of NCP as ∆DOC.

Highlights

Phytoplankton blooms spanning the subtropical to the polar latitudes of the North Atlantic occur annually and are central to biogeochemical cycling in the global ocean (Duursma, 1963; Lochte et al, 1993; Sieracki et al, 1993; Carlson et al, 1998; Falkowski, 1998; Behrenfeld, 2010)
When photoautotrophy exceeds net heterotrophic processes within the surface layer, the seasonal net community production (NCP, moles C per unit volume or area per time) can be dissolved organic carbon (DOC) Accumulation and Vertical Export estimated from the biological production of oxygen (Plant et al, 2016) or the net drawdown of total carbon dioxide or nitrate as it is fixed to organic matter (Codispoti et al, 1986; Hansell et al, 1993; Hansell and Carlson, 1998)
Organic matter resulting from NCP has three main fates: (1) accumulation as particulate organic carbon (POC) in the surface layer followed by export via the passive sinking flux (McCave, 1975), (2) export from the surface layer via vertical migrating zooplankton (Steinberg et al, 2000), and (3) accumulation as suspended organic matter [i.e., dissolved organic carbon (DOC) and suspended POC (POCs)] in the surface layer followed by export via physical transport (Carlson et al, 1994; Hansell and Carlson, 1998; Sweeney et al, 2000; Dall’Olmo et al, 2016)

Summary

Introduction

Phytoplankton blooms spanning the subtropical to the polar latitudes of the North Atlantic occur annually and are central to biogeochemical cycling in the global ocean (Duursma, 1963; Lochte et al, 1993; Sieracki et al, 1993; Carlson et al, 1998; Falkowski, 1998; Behrenfeld, 2010). These blooms are net autotrophic events initiated by an imbalance between phytoplankton division and loss rates, created by favorable abiotic conditions for incident sunlight and subsurface attenuation, surface mixing layer dynamics, nutrients, and temperature (Behrenfeld and Boss, 2018). We refer to the seasonal accumulation rate of surplus surface layer DOC as DOC (vertically integrated moles C m−2 time period−1 or moles C L−1 time period−1)

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